Operating System Short Question And Answer Set-5

1.What are the various layers of a file system?
The file system is composed of many different levels. Each level in the design uses the feature of the lower levels to create new features for use by higher levels.

  • Application programs
  • Logical file system
  • File-organization module
  • Basic file system
  • I/O control
  • Devices

2.What are the structures used in file-system implementation?
Several on-disk and in-memory structures are used to implement a file system

a. On-disk structure include

  • Boot control block
  • Partition block
  • Directory structure used to organize the files
  • File control block (FCB)

b. In-memory structure include

  • In-memory partition table
  • In-memory directory structure
  • System-wide open file table
  • Per-process open table

3.What are the functions of virtual file system (VFS)?
a. It separates file-system-generic operations from their implementation defining a clean VFS interface. It allows transparent access to different types of file systems mounted locally.

b. VFS is based on a file representation structure, called a vnode. It contains a numerical value for a network-wide unique file .The kernel maintains one vnode structure for each active file or directory.

4.Define seek time and latency time.
The time taken by the head to move to the appropriate cylinder or track is called seek time. Once the head is at right track, it must wait until the desired block rotates under the read-write head. This delay is latency time.

5.What are the allocation methods of a disk space?
Methods of allocating disk space which are widely in use are
a. Contiguous allocation
b. Linked allocation
c. Indexed allocation

6.What are the advantages of Contiguous allocation?
The advantages are
a. Supports direct access
b. Supports sequential access
c. Number of disk seeks is minimal.

7.What are the drawbacks of contiguous allocation of disk space?
The disadvantages are
a. Suffers from external fragmentation
b. Suffers from internal fragmentation
c. Difficulty in finding space for a new file
d. File cannot be extended
e. Size of the file is to be declared in advance

8.What are the advantages of Linked allocation?
The advantages are
a. No external fragmentation
b. Size of the file does not need to be declared

9.What are the disadvantages of linked allocation?
The disadvantages are
a. Used only for sequential access of files.
b. Direct access is not supported
c. Memory space required for the pointers.
d. Reliability is compromised if the pointers are lost or damaged

10.What are the advantages of Indexed allocation?
The advantages are
a. No external-fragmentation problem
b. Solves the size-declaration problems.
c. Supports direct access

11.How can the index blocks be implemented in the indexed allocation scheme?
The index block can be implemented as follows
a. Linked scheme
b. Multilevel scheme
c. Combined scheme

12.Define rotational latency and disk bandwidth.
Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. The disk bandwidth is the total number of bytes transferred, divided by the time between the first request for service and the completion of the last transfer.

13.How free-space is managed using bit vector implementation?
The free-space list is implemented as a bit map or bit vector. Each block is represented by 1 bit. If the block is free,the bit is 1; if the block is allocated, the bit is 0.

14.Define buffering.
A buffer is a memory area that stores data while they are transferred between two devices or between a device and an application. Buffering is done for three reasons
a. To cope with a speed mismatch between the producer and consumer of a data stream
b. To adapt between devices that have different data transfer sizes
c. To support copy semantics for application I/O

15.Define caching.
A cache is a region of fast memory that holds copies of data. Access to the cached copy is more efficient than access to the original. Caching and buffering are distinct functions, but sometimes a region of memory can be used for both purposes.

16.Define spooling.
A spool is a buffer that holds output for a device, such as printer, that cannot accept interleaved data streams. When anapplication finishes printing, the spooling system queues the corresponding spool file for output to the printer. The spooling system copies the queued spool files to the printer one at a time.

17.What are the various disk-scheduling algorithms?
The various disk-scheduling algorithms are
a. First Come First Served Scheduling
b. Shortest Seek Time First Scheduling
c. SCAN Scheduling
d. C-SCAN Scheduling
f. LOOK scheduling

18.What is low-level formatting?
Before a disk can store data, it must be divided into sectors that the disk controller can read and write. This process is called low-level formatting or physical formatting. Lowlevel formatting fills the disk with a special data structure for each sector. The data structure for a sector consists of a header,a data area, and a trailer.

19.What is the use of boot block?
For a computer to start running when powered up or rebooted it needs to have an initial program to run. This bootstrap program tends to be simple. It finds the operating system on the disk loads that kernel into memory and jumps to an initial address to begin the operating system execution. The full bootstrap program is stored in a partition called the boot blocks, at fixed location on the disk. A disk that has boot partition is called boot disk
or system disk.

20.What is sector sparing?
Low-level formatting also sets aside spare sectors not visible to the operating system. The controller can be told to replace each bad sector logically with one of the spare sectors.This scheme is known as sector sparing or forwarding.